Solution for this issue is that, you have to add the app to battery optimization as "Not to Optimise". And Soft reset your phone.
Even Alarm app issue can do the same.
I had same issue, I tried this on my Nokia 6, now it works fine for me.
Yes you can find battery optimisation settings as follows:
Settings - Apps - tap on the round settings icon - special access - battery optimisation"
Then you can select these three apps (Phone, Alarm & Message" as "Not to Optimise"
Hope this works for you.